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474                               CHEMICAL ENGINEERING
case of each type of refractory. At the same time we shall consider the methods employed for estimating these special properties since familiarity with them will enable us to differentiate the various materials and to recognize their limitations.
The principal factors involved in the use of refractories may be summarized as follows:
1.  Chemical and mineralogical composition.
2.  Refractoriness.
3.  Permanent expansion or contraction upon reheating.
4.  Porosity and specific gravity.
5.  Resistance to compression at furnace temperatures.
6.  Thermal properties, heat capacity, thermal conductivity and expansion.
7.  Resistance to sudden temperature changes.
8.  Resistance to slagging action.
9.  Mechanical strength and resistance to abrasion. 10. Electrical resistance and dielectric strength.
Chemical Composition.—The classification of refractories for metallurgical purposes has been principally based on the composition of the materials as expressed by their acidity, i.e., the ratio of silica to the basic constituents. It was assumed that siliceous slags would not attack refractories high in silica, and basic slags would not combine with basic refractories. In a general way this assumption is undoubtedly true, but it must be realized that such properties as density are of great importance in this respect. Porous refractories, no matter how well adapted they may be from the chemical standpoint to resist certain slags, are poorly suited for such requirements since they absorb any liquid by capillary action and hence are destroyed in a short time. On the other hand, in the calcination of solids, an excellent showing may be made by refractories which represent, chemically, exactly the opposite characteristics of the charge. Thus in the burning of lime, silica brick or quartzite kiln linings have given very excellent results. This obviously is due to the small reacting surfaces offered by the limestone and the comparatively low temperature involved.
At high temperatures the reaction between solids is much more active and diffusion phenomena more rapid. In the open-hearth furnace, therefore, it is necessary to separate the acid silica brick from the basic magnesite by means of a neutral parting of chromite.
It is evident that chemical composition is of fundamental importance in determining the character and quality of a refractory. A low content of basic impurities in siliceous materials and of acid ones in basic refractories is obviously desirable from the standpoint of heat resistance and for this purpose chemical analysis is of great importance in determining the causes of success or failure. In refractories used at temperatures, close to their softening point the presence of impurities amounting only to 1 or 2 per cent may cause failure. The chemical analysis of refractory materials requires careful work and the modem analytical methods proposed by Hillebrand, the Geophysical Laboratory of the Carnegie Institution, Mellor, and others should be employed.
The range of materials used as refractories is very wide and includes such substances as the fireclays and kaolins, quartzites, bauxites, schist, magnesite, dolomite, graphite, carborundum, fused alumina, chromite, magnesium aluminate, magnesium silicate, zirconium oxide, zirconium silicate, boron nitride, and others.
The results of the chemical analysis are sometimes computed in the form of empirical formulas which simplify comparison of the materials with recognized standards